Legislations on vehicle and heavy goods vehicle emissions stipulate, amongst other things, a reduction in the release of nitrogen oxides NOx into the atmosphere. One known way to achieve this objective is to use the SCR (Selective Catalytic Reduction) process which enables the reduction of nitrogen oxides by injection of a reducing agent, generally ammonia, into the exhaust line. This ammonia may derive from the pyrolytic decomposition of an ammonia precursor solution, whose concentration may be the eutectic concentration. Such an ammonia precursor is generally a urea solution.
With the SCR process, the high levels of NOx produced in the engine during combustion at optimized efficiency are treated in a catalyst on exiting the engine. This treatment requires the use of the reducing agent at a precise concentration and of extreme quality. The solution is thus accurately metered and injected into the exhaust gas stream where it is hydrolysed before converting the nitrogen oxide (NOx) to nitrogen (N2) and water (H2O).
In order to do this, it is necessary to equip the vehicles with a tank containing an additive (generally urea) solution and also a device for metering the desired amount of additive and injecting it into the exhaust line.
Given that the aqueous urea solution generally used for this purpose (eutectic solution containing 32.5 wt % of urea) freezes at −11° C., it is necessary to provide a heating device to liquefy the solution in order to be able to inject it into the exhaust line in the event of starting in freezing conditions.
Moreover, with a view to facilitating the operation, the re-starting of the system in case of freezing and to prevent the often flexible tubing that transports the urea from bursting, it is advantageous to purge the conduits after operating the system. In particular, it is advantageous to purge the line connecting the additive tank to the injector (or injection line) and the return line, where appropriate.
Several systems have been provided in the prior art for this purpose.
Thus, Application WO 2006/064028 in the name of the Applicant proposes an SCR system where purging of the line(s) takes place by sucking the exhaust gases through the injector using the pump of the system, which for this purpose either rotates in the opposite direction or is associated with a valve that makes it possible to reverse the flow. The advantage of this system is its simplicity and the reduced number of modifications that must be made to it in order to add this function (this is because it suffices to choose a pump that rotates in both directions or to provide a 4-way valve in the system).
This document also mentions the fact that it is preferable not to purge directly after shutting off the engine as the gases might be too hot and might damage certain parts of the system. For this purpose it recommends a temperature control in order to adjust the time delay before purging.
However, it would appear that this problem is not, in reality, truly critical (no doubt because as soon as the engine is shut off, the temperature in the exhaust pipe drops rapidly) and that, in addition, it may be prevented by choosing to suck up air from outside the exhaust pipe (and not the air/gases in said pipe). Conversely, if the purge is started immediately after the engine is shut off, there is a risk of spraying urea into the exhaust pipe and/or over the catalyst. This phenomenon is capable of causing the premature ageing of the catalyst and also corrosion problems in the exhaust pipe following the release of ammonia.